Google recently stopped development on Notebook, which means it is no longer being improved upon or open to sign-ups by new users.

If you're visiting notebook for the first time, instead try exploring other Google products that are still supported. For example, if you're trying to jot down some quick notes or create a document that you can share with others, check out Google Docs. And Google Bookmarks lets you remember web pages that you liked and access them easily.

If you're an existing Google Notebook user - don't worry! You can still access all your notebooks and data from www.google.com/notebook, as always. Just sign out and log back in with the username you used to create your notes.

For more information, take a look at the announcement on the Google Notebook Blog.